9.3 unexpand: Convert spaces to tabs

unexpand writes the contents of each given file, or
standard input if none are given or for a file of ‘-’, to
standard output, converting blanks at the beginning of each line into
as many tab characters as needed. In the default POSIX
locale, a blank is a space or a tab; other locales may specify
additional blank characters. Synopsis:

unexpand [option]... [file]...

By default, unexpand converts only initial blanks (those
that precede all non-blank characters) on each line. It
preserves backspace characters in the output; they decrement the column
count for tab calculations. By default, tabs are set at every 8th
column.

If only one tab stop is given, set the tabs tab1 columns apart
instead of the default 8. Otherwise, set the tabs at columns
tab1, tab2, ... (numbered from 0), and leave blanks
beyond the tab stops given unchanged. Tab stops can be separated by
blanks as well as by commas. This option implies the -a option.

For compatibility, GNU unexpand supports the obsolete option syntax,
-tab1[,tab2]..., where tab stops must be
separated by commas. (Unlike -t, this obsolete option does
not imply -a.) New scripts should use --first-only -t
tab1[,tab2]... instead.

‘-a’

‘--all’

Also convert all sequences of two or more blanks just before a tab stop,
even if they occur after non-blank characters in a line.

An exit status of zero indicates success,
and a nonzero value indicates failure.